1. Field of the Invention
The present invention relates to a magneto-optical recording medium used for recording or reproducing information, a method for producing the same, and an apparatus for producing the same.
2. Description of the Related Art
As a repeatedly rewritable recording medium having a high density, a magneto-optical recording medium and a recording/reproducing apparatus for recording a minute domain onto a magnetic thin film with thermal energy of laser light, and reproducing a signal using a magneto-optical effect are being developed actively. In such a magneto-optical recording medium, when the diameter and interval of recording bits (domains for recording) become smaller with respect to the diameter of a light beam focused onto the medium, reproduction characteristics are degraded. This is caused as follows: an adjacent recording bit enters the diameter of a light beam focused onto an intended recording bit, which makes it difficult to reproduce information from individual recording bits separately.
In order to solve the above-mentioned problem, attempts have been made to enhance a recording density by modifying the configuration of a recording medium and a reproducing method. For example, a super-resolution system, a domain wall displacement detection (DWDD) reproducing system using the displacement of a domain wall, and the like have been proposed. Herein, a DWDD reproducing system disclosed in JP6(1997)-290496 A will be described with reference to FIG. 9.
In a magneto-optical recording medium shown in FIG. 9, a reproducing layer (domain wall displacement layer) 91, an intermediate layer (switching layer) 92, and a recording layer 93 that constitute magnetic layers 90 are exchange-coupled to each other, and a minute recording domain 96 of the recording layer 93 is enlarged in the reproducing layer 91, whereby an amplitude of a reproducing signal is increased, making it possible to conduct high-density recording. Arrows represent the sublattice magnetization directions of transition metal in each layer. In each layer, a domain wall 94 is formed between domains in which magnetization directions are opposite to each other. A region 95 of the intermediate layer 92 reaches a temperature equal to or higher than a Curie temperature due to the irradiation with laser light for reproduction, whereby a magnetic order is lost.
The conditions desired for the above-mentioned magneto-optical recording medium are summarized by the following four points:
(1) The magneto-optical recording medium has the recording layer 93 that holds minute domains stably in a range from a room temperature to a reproducing temperature.
(2) Even when the magneto-optical recording medium is heated to the vicinity of a Curie temperature of the intermediate layer 92, the reproducing layer 91, the intermediate layer 92, and the recording layer 93 are exchange-coupled to each other.
(3) When the intermediate layer 92 reaches a temperature exceeding its Curie temperature so as to lose its magnetic order, exchange coupling between the recording layer 93 and the reproducing layer 91 is cut off.
(4) The domain wall coercive force of the reproducing layer 91 is small, and a domain wall energy gradient is caused by a temperature gradient. Therefore, in a region of the reproducing layer 91 where exchange coupling is cut off by the intermediate layer 92, the domain wall 94 is displaced from a position transferred from a domain of the recording layer 93. As a result, the magnetization in this region is aligned in the same direction, and an interval (recording mark length) between the magnetic walls 94 of the recording layer 93 is enlarged.
In FIG. 9, when the magneto-optical recording medium is moved (rotated in the case of a disk) in the right direction on the drawing surface while laser light is radiated thereto, due to the high linear velocity of the medium, the position at which a film temperature becomes maximum is placed on the backward side from the center of a beam spot in a traveling direction (left direction on the drawing surface) thereof. A domain wall energy density σ1 in the reproducing layer 91 generally decreases with an increase in temperature to become 0 at a temperature equal to or higher than a Curie temperature. Therefore, in the presence of a temperature gradient, the domain wall energy density σ1 is decreased toward a higher temperature side.
Herein, a force F1 represented by the following expression acts on a domain wall present at a position “x” in a medium movement direction (circumferential direction of a disk).F1∝−dσ1/dx
The force F1 acts so as to move a domain wall in a direction of lower domain wall energy. In the reproducing layer 91, a domain wall coercive force is smaller and a domain wall mobility is larger compared with those of the other magnetic layers. Therefore, when exchange coupling from the intermediate layer 92 is cut off, a domain wall moves very rapidly in a direction of lower domain wall energy due to the force F1.
Referring to FIG. 9, in a region of the medium before being irradiated with laser light (e.g., a region at a room temperature), three magnetic layers are exchange-coupled to each other, and domains recorded on the recording layer 93 are transferred to the reproducing layer 91. In this state, the domain wall 94 is present between domains having magnetization directions opposite to each other in each layer. In the region 95 that reaches a temperature equal to or higher than the Curie temperature of the intermediate layer 92 due to the irradiation with laser light, magnetization of the intermediate layer 92 is lost, and the exchange coupling between the reproducing layer 91 and the recording layer 93 is cut off. Therefore, a force for holding a domain wall is lost in the reproducing layer 91, and a domain wall is displaced to a higher temperature side due to the force F1 applied to the domain wall. At this time, a domain wall displacement speed is sufficiently higher than that of the medium movement speed. Thus, a domain larger than a domain stored on the recording layer 93 is transferred to the reproducing layer 91.
In a magneto-optical recording medium using the DWDD reproducing system, for the purpose of displacing a domain wall easily, the following is proposed: guide grooves having a rectangular cross-section are formed on a substrate so that domain walls are not generated on the side of the recording tracks, whereby the respective tracks are separated by the grooves. However, even if guide groove having a rectangular cross-section are formed, films actually are accumulated to some degree in stepped portions, and magnetic layers are connected to each other. As a result, magnetic separation cannot be conducted completely, which inhibits the displacement of a domain wall.